Polymer buffer layers and their use in light-emitting diodes

ABSTRACT

The present invention is directed to improved transparent electrically conductive buffer layers for light-emitting diodes. The use of an aqueous dispersion containing an electrically conducting polymer and a non-ionic surfactant or an ionic fluorosurfactant to form the buffer layer has been shown to provide a buffer layer that results in improved light-emitting diode performance.

This application is a divisional of U.S. applicaton, Ser. No.10,372,351, filed Feb. 24, 2003, now U.S. Pat. No. 6,955,773 issued Oct.18, 2005, which is a Non- Provisional of U.S. application No.60/361,029, filed Feb. 28, 2002.

FIELD OF THE INVENTION

This invention relates to a transparent electrically conductive bufferlayer comprised of a conductive polymer and a surfactant and tolight-emitting devices containing such a layer.

BACKGROUND OF THE INVENTION

Electrically conductive polymers have been found to be useful in organicelectronic devices that emit light, such as light-emitting diodes(LEDs). In all such devices an organic active emission layer issandwiched between two electrodes which serve as the anode and thecathode. At least one of the electrodes is comprised of a material thattransmits light. The active emission layer emits light through thelight-transmitting electrode, typically an indium tin oxide anode, uponapplication of a voltage to the device. It is well known to use a layerof conductive polymer, such as a polyaniline or a poly(dioxythiophene),between the inorganic anode and the light-emitting layer. The conductivepolymer layer is variously referred to as part of the anode, ahole-injection layer or a buffer layer. Such systems have been describedin, for example, Jonas et al., U.S. Pat. No. 5,766,515, Yang, U.S. Pat.No. 5,723,873 and Zhang et al., U.S. Pat. No. 5,798,170.

Useful synthetic procedures for the preparation of a polyaniline (PAni)or a poly(dioxythiophene) such as poly(3,4-ethylenedioxythiophene)(PEDOT) are well known and these materials are readily availablecommercially.

Kinlen et al., U.S. Pat. No. 5,840,214, disclose a process forincreasing the conductivity of a polyaniline comprising contacting thepolyaniline with an ionic surfactant. Kinlen et al. demonstrated theprocess by forming an polyaniline film and subsequently contacting thepolyaniline film with an ionic surfactant. They also disclose that thetreatment of the polyaniline salt in xylenes prior to processing intothe final form is possible where both the polyaniline salt and theanionic surfactant are soluble in an amount of at least about 1% w/w foreach of the polyaniline salt and the ionic surfactant.

It is desirable to find a polymer with electrical properties thatprovides better performance as a buffer layer in a light-emittingdevice.

SUMMARY OF THE INVENTION

The present invention provides a light-emitting device on a substrate.The light-emitting device comprises an anode, a cathode, an activeemission layer positioned between the anode and the cathode, and abuffer layer positioned between the anode and the active emission layer,wherein the buffer layer is comprised of an electrically conductingpolymer and a surfactant. The amount of the surfactant is from about 0.5wt % to about 15 wt % of the total weight of the surfactant and theelectrically conducting polymer and the surfactant is selected from afluorosurfactant, a polyoxyethylene alkylphenol ether, or a mixturethereof.

This invention also provides a light-emitting device on a substrate, thelight-emitting device comprising an anode, a cathode, an active emissionlayer positioned between the anode and the cathode, and a buffer layerpositioned between the anode and the active emission layer, wherein thebuffer layer is formed using an aqueous dispersion of an electricallyconducting polymer to which has been added a surfactant. The dispersionis characterized by a surface tension of between 25 and 35 mN/M. Theconcentration of the electrically conducting polymer is from 0.1 to 5.0%w/w of the total dispersion. The concentration of the surfactant is lessthan about 0.4% w/w of the total dispersion and the surfactant isselected from a fluorosurfactant, a polyoxyethylene alkylphenol ether,or a mixture thereof.

The use of the buffer layer of this invention also enables the use of alight-transmitting bilayer anode comprised of a metal layer and a thinlayer of an inorganic dielectric with the dielectric layer adjacent tothe buffer layer.

This invention also provides a composition useful for forming a bufferlayer positioned between an anode and an active emission layer of alight-emitting device, the composition comprising an aqueous dispersionof an electrically conducting polymer to which has been added asurfactant. The dispersion is characterized by a surface tension ofbetween 25 and 35 mN/M. The concentration of the electrically conductingpolymer is from 0.1 to 5.0% w/w of the total dispersion. Theconcentration of the surfactant is less than about 0.4% w/w of the totaldispersion and the surfactant is selected from a fluorosurfactant, apolyoxyethylene alkylphenol ether, or a mixture thereof.

As used herein, the term “total dispersion” indicates the total of thesolids and the liquid carrier of the dispersion. In addition, the IUPACnumbering system is used throughout, where the groups from the PeriodicTable are numbered from left to right as 1 through 18 (CRC Handbook ofChemistry and Physics, 81^(st) Edition, 2000). The phrase “adjacent to”does not necessarily mean that one layer is immediately next to anotherlayer. An intermediate layer or layers may be provided between layerssaid to be adjacent to each other.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention relates to the formation of a layer of conductive polymerthat provides better performance as a buffer layer in a light-emittingdevice. It has been found that a buffer layer formed using a compositioncomprised of an electrically conductive polymer and specificsurfactants, and therefore comprised of these same constituents,provides a buffer layer with the desired improved performance.Surfactants could be considered impurities that adversely impact deviceperformance. Surprisingly, these specific surfactants i.e.,fluorosurfactants or polyoxyethylene alkylphenol ethers, not onlyfacilitate the coating of the aqueous dispersion to form the bufferlayer, but also improve device performance and facilitate the use offlexible substrates. It is critical to evaluate a buffer layer as partof a light-emitting device in order to demonstrate improved performance.

The present invention also relates to an light-emitting devicecomprising an organic active emission layer sandwiched between twoelectrical contact layers, wherein a buffer layer comprising anelectrically conducting polymer and a surfactant is positioned betweenthe light-emitting layer and the electrical contact layer whichfunctions as an anode. The device has an inorganic anode layer and acathode layer. Adjacent to the anode is the buffer layer. Adjacent tothe cathode is an optional layer comprising an electron transportmaterial. Between the buffer layer and the cathode (or optional electrontransport layer) is the active emission layer.

The device generally also includes a support, which can be adjacent tothe anode or the cathode. Most frequently, the support is adjacent tothe anode. The support can be flexible or rigid, organic or inorganic.Generally, glass or flexible organic films, such as poly(ethyleneterephthalate), are used as a support.

The anode is an electrode that is particularly efficient for injectingor collecting positive charge carriers. The anode can be a metal, amixed metal, an alloy, a metal oxide or a mixed-metal oxide. Suitablemetals include the Group 11 metals, the metals in Groups 4, 5, and 6,and the Group 8–10 transition metals. If the anode is to belight-transmitting, mixed-metal oxides of Groups 12, 13 and 14 metals,such as indium tin oxide (ITO), are generally used.

The use of the buffer layer of this invention also enables the use of alight-transmitting bilayer anode comprised of a metal layer and a thinlayer of an inorganic dielectric with the dielectric layer adjacent tothe buffer layer. An additional thin dielectric layer can be depositedon the substrate before the metal layer is deposited. A multilayer anodecomprised of two or more such bilayers of metal and dielectric, i.e.,metal-dielectric-metal-dielectric, etc., can also be used. The metal inthe bilayer is comprised of silver, gold, nickel, chromium, cobalt,alloys thereof and alloys of these metals with iron. The dielectric inthe bilayer is comprised of an oxide of a Group 13 or 14 element, suchas silicon, indium, titanium, tin, germanium and aluminum, or a fluoridesuch as magnesium fluoride. In a preferred embodiment, the buffer layeris provided directly on a dielectric layer of a multilayer anodecontaining at least one bilayer.

The anode is usually applied by a physical vapor deposition process. Theterm “physical vapor deposition” refers to various deposition approachescarried out in vacuo. Thus, for example, physical vapor depositionincludes all forms of sputtering, including ion beam sputtering, as wellas all forms of vapor deposition such as electron beam evaporation. Aspecific form of physical vapor deposition that is useful is radiofrequency magnetron sputtering.

The buffer layer can be applied using any conventional means, includingspin-coating, casting, and printing, such as gravure printing. Thebuffer layer can also be applied by ink jet printing or thermalpatterning. The composition of this invention that is useful for forminga buffer layer positioned between the anode and the active emissionlayer of a light-emitting device comprises an aqueous dispersion of anelectrically conducting polymer to which has been added a surfactant.The electrically conducting polymer is preferably a polyaniline (PAni)or a poly(dioxythiopene). A preferred poly(dioxythiopene) ispoly(3,4-ethylenedioxythiophene) (PEDOT). The oxidative polymerizationprocesses for producing a polyaniline or a poly(dioxythiopene) can becarried out in the presence of a poly(acrylamidoalkylsulfonic acid)(PAAMPSA) or a poly(styrenesulfonic acid) (PSSA) and result in complexessuch as PEDOT/PAAMPSA, PEDOT/PSSA, Pani/PAAMPSA and PAni/PSSA. As usedherein, the terms polyaniline, poly(dioxythiopene) andpoly(3,4-ethylenedioxythiophene) each means the polymer itself as wellas the complexes comprised of the polymer.

The surfactant is a fluorosurfactant or a polyoxyethylene alkylphenolether. The surfactant can be a non-ionic surfactant, i.e., a non-ionicfluorosurfactant or a polyoxyethylene alkylphenol ether or an ionicfluorosurfactant. Non-ionic surfactants are preferred. A preferredpolyoxyethylene alkylphenol ether is polyoxyethylene (10) isooctylphenylether. The non-ionic fluorosurfactant is preferably a fluoroaliphaticpolymeric ester or a fluorosurfactant containing a perfluoroalkyl groupwith the formula F(CF₂CF₂)₃₋₈. Especially preferred is aperfluoroalkylpolyoxyethylene. An ionic fluorosurfactant preferablycontains a perfluoroalkyl group with the formula F(CF₂CF₂)₃₋₈.Especially preferred is a lithium carbonate ofperfluoroalkylethylenesulfurethylene anionic surfactant. Amphotericsurfactants such as perfluoroalkyl N-type betaines, and cationicsurfactants such as perfluoroalkylammonium halides, can also be used.

The concentration of the electrically conducting polymer is from 0.1 to5.0% w/w of the total dispersion. The concentration of the surfactant isless than about 0.4% w/w of the total dispersion. Preferably, theconcentration of the electrically conducting polymer is from 0.5 to 3.0%w/w of the total dispersion and the concentration of the surfactant isless than about 0.3% w/w of the total dispersion. The amount of aparticular surfactant used depends on the particular electricallyconducting polymer used and the concentration of that electricallyconducting polymer in the dispersion. The amount of the surfactant ischosen to provide a surface tension of the dispersion of between 25 and35 mN/M.

This invention provides a light-emitting device in which the bufferlayer is made using the composition that comprises an aqueous dispersionof an electrically conducting polymer to which has been added asurfactant as described in the previous paragraph.

The buffer layer of the light-emitting device of the invention iscomprised of an electrically conducting polymer and a surfactant whereinthe amount of the surfactant is from about 0.5 wt % to about 15 wt % ofthe total weight of the surfactant and the electrically conductingpolymer. The surfactant is a fluorosurfactant or a polyoxyethylenealkylphenol ether as described in connection with the composition of theinvention. Preferably the amount of the surfactant is from about 1 wt %to about 10 wt % of the total weight of the surfactant and theelectrically conducting polymer.

In general, the anode and the conductive polymer layer will bepatterned. It is understood that the pattern may vary as desired. Thelayers can be applied in a pattern by, for example, positioning apatterned mask or photoresist on the first flexible composite barrierstructure prior to applying the first electrical contact layer material.Alternatively, the layers can be applied as an overall layer andsubsequently patterned using, for example, a photoresist and wetchemical etching. As discussed above, the buffer layer can also beapplied in a pattern by ink jet printing, lithography or thermaltransfer patterning. Other processes for patterning that are well knownin the art can also be used.

In a light-emitting diode, the active emission layer emits light whensufficient bias voltage is applied to the electrical contact layers,i.e., the anode and cathode. The light-emitting active emission layermay contain any organic electroluminescent or other organiclight-emitting materials. Such materials can be small molecule materialssuch as those described in, for example, Tang, U.S. Pat. No. 4,356,429,Van Slyke et al., U.S. Pat. No. 4,539,507, the relevant portions ofwhich are incorporated herein by reference. LEDs with an active emissionlayer comprised of small molecule (SMO) materials are common referred toas SMOLEDs. Alternatively, such materials can be polymeric materialssuch as those described in Friend et al. (U.S. Pat. No. 5,247,190),Heeger et al. (U.S. Pat. No. 5,408,109), Nakano et al. (U.S. Pat. No.5,317,169), the relevant portions of which are incorporated herein byreference. LEDs with an active emission layer comprised of polymeric (P)materials are common referred to as PLEDs. Preferred electroluminescentmaterials are semiconductive conjugated polymers. An example of such apolymer is poly(p-phenylenevinylene) referred to as PPV. Thelight-emitting materials may be dispersed in a matrix of anothermaterial, with and without additives, but preferably form a layer alone.The active organic layer generally has a thickness in the range of50–500 nm. In order to prevent cross-talk between lines or pixels of thepatterned anode, electrical conductivity of the buffer layers should beas low as possible without jeopardizing the light emission properties ofthe device. It has been found that when the buffer layer comprises anelectrically conducting polymer and a surfactant that is afluorosurfactant or a polyoxyethylene alkylphenol ether, thelight-emitting diode shows a higher light emission efficiency,equivalent or lower turn-on voltage, and equivalent or higher brightnessas compared to when the buffer layer comprises the electricallyconducting polymer and no surfactant.

The active emission layer containing the active organic material can beapplied from solutions by any conventional means, includingspin-coating, casting, and printing. The active organic materials can beapplied directly by vapor deposition processes, depending upon thenature of the materials. It is also possible to apply an active polymerprecursor and then convert to the polymer, typically by heating.

The cathode is an electrode that is particularly efficient for injectingor collecting electrons or negative charge carriers. The cathode can beany metal or nonmetal having a lower work function than the firstelectrical contact layer (in this case, an anode). Materials for thesecond electrical contact layer can be selected from alkali metals ofGroup 1 (e.g., Li, Cs), the Group 2 (alkaline earth) metals, the Group12 metals, the lanthanides, and the actinides. Materials such asaluminum, indium, calcium, barium, and magnesium, as well ascombinations, can be used.

The cathode layer is usually applied by a physical vapor depositionprocess. In general, the cathode layer will be patterned, as discussedabove in reference to the anode layer and the buffer layer. Similarprocessing techniques can be used to pattern the cathode layer.

The optional layer between the cathode and the active emission layer canfunction both to facilitate electron transport, and also serve as aconfinement layer to prevent quenching reactions at layer interfaces.Preferably, this layer promotes electron mobility and reduces quenchingreactions. Examples of electron transport materials for the optionallayer include metal chelated oxinoid compounds, such astris(8-hydroxyquinolato)aluminum (Alq₃); phenanthroline-based compounds,such as 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (DDPA) or4,7-diphenyl-1,10-phenanthroline (DPA), and azole compounds such as2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole (PBD) and3-(4-biphenylyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4-triazole (TAZ).

It is known to have other layers in organic electronic devices. Forexample, there can be a layer between the buffer layer and the activeemission layer to facilitate positive charge transport and/or band-gapmatching of the layers, or to function as a protective layer. Similarly,there can be additional layers between the active emission layer and thecathode layer to facilitate negative charge transport and/or band-gapmatching between the layers, or to function as a protective layer.Layers that are known in the art can be used. In addition, any of theabove-described layers can be made of two or more layers. Alternatively,some or all of the anode layer, the buffer layer, the active emissionlayer, and the cathode layer, may be surface treated to increase chargecarrier transport efficiency. The choice of materials for each of thecomponent layers is preferably determined by balancing the goals ofproviding a device with high device efficiency.

The device can be prepared by sequentially depositing the individuallayers on a suitable substrate. Substrates such as glass and polymericfilms can be used. In most cases the anode is applied to the substrateand the layers are built up from there. However, it is possible to firstapply the cathode to a substrate and add the layers in the reverseorder. In general, the different layers will have the following range ofthicknesses: anode, 50–500 nm, preferably 100–200 nm; buffer layer,5–250 nm, preferably 20–200 nm; light-emitting active emission layer,1–100 nm, preferably 10–80 nm; optional electron transport layer, 5–100nm, preferably 20–80 nm; cathode, 20–1000 nm, preferably 30–500 nm.

EXAMPLES

The following examples illustrate certain features and advantages of thepresent invention. They are intended to be illustrative of theinvention, but not limiting. All percentages are by weight, unlessotherwise indicated.

All measurements of surface tension reported herein were made with aDigital Krüss Process Tensiometer K12 (Krüss GmbH, Hamburg, Germany).Measurements were carried out at room temperature. The vessel containingliquid to be measured has a diameter of 6.65 cm and a height of 10.0 cm.The measurement method is the Krüss 11 (P/SFT) with a minimum standarddeviation of 0.02 mN/m.

Light emission measurement performed on flexible light-emitting diodes(LEDs), i.e., light-emitting diodes on flexible substrates, reportedherein were carried out in the following manner. A flexible LED samplefor the light emission measurement consisted of a flexible substrate, alayer of indium-tin-oxide to serve as the anode, a buffer layer, a layerof active emission material, and an array of aluminum dots 1.0 mm indiameter to serve as the cathode. The programmable voltage source of anHP 4155A semiconductor parameter analyzer (Hewlett-Packard, Palo Alto,Calif.) was connected to the anode and one aluminum dot with amechanical device providing pressure contacts to the electrodes. Lightemission was detected with a photodiode and thereby converted to acurrent for recording the light emission intensity. Device current andphotodiode current were recorded continuously as the voltage applied tothe electrodes was being stepped up automatically.

Light emission measurement performed on rigid light-emitting diodes(LEDs), i.e., light-emitting diodes on rigid, e.g., glass, substrates,reported herein were carried out in the following manner. A rigid LEDsample for the light emission measurement was made as described for theflexible LEDs except that the substrate consisted of a rigid (glass)substrate. The current vs. the voltage, the light emission intensity(brightness) vs. the voltage, and efficiency (Cd/A) were measured with aKeithley 236 source-measure unit (Keithley Instrument Inc., Cleveland,Ohio), and a S370 optometer with a calibrated silicon photodiode (UDTSensor, Inc. Hawthorne, Calif.).

Unless otherwise indicated, all measurements were carried out at roomtemperature, about 20°.

Comparative Experiment A

This Comparative Experiment demonstrates the results obtained whenforming a buffer layer by spin-coating an aqueous polyaniline dispersionwith no surfactant onto a flexible substrate containing an indium tinoxide (ITO) anode in an attempt to make a light-emitting diode.

The flexible substrate used in this Comparative Experiment is OC50/ST504PET 7-mil (180 μm) poly(ethylene terephthalate) sheet, on one side ofwhich has been deposited a thin layer of 50 ohm per square ITO, obtainedfrom CPFilms Inc. (Canoga Park; CA). A 6 inch×1 inch (15.2 cm×2.5 cm)strip of the PET with the ITO layer was cut into 1 inch (2.5 cm)squares.

These squares were then cleaned and dried in the following manner. Theywere first cleaned with a dilute deionized water solution of Liqui-Nox®from Alonox, Inc. (New York, N.Y.) in a sonication bath for one hour.The squares were placed on a rack and were then rinsed with deionizedwater. They were placed into the sonication bath with fresh deionizedwater and resonicated for one hour. The squares were then removed fromthe rack and rinsed individually with deionized water first and thenwith isopropyl alcohol. The squares were then replaced into the rack andthe rack placed into a vacuum oven at room temperature, about 20° C.,overnight.

Spin-coating of an aqueous polyaniline dispersion was conducted asfollows. A microscope slide was placed on a chuck equipped on aspin-coater from Headway Research Inc. (Garland, Tex.). A tiny drop ofwater was placed on the center of the slide to keep a square in placewhen placed on the slide. The surface of the square with the ITO layerwas facing up and it was onto this ITO layer that the aqueouspolyaniline dispersion was placed. The aqueous polyaniline dispersionwas Ormecon® D1002W, which contains 2.6% w/w polyaniline in water,obtained from Ormecon Chemie Gmbh & Co, KG (Aammersbek, Germany). Thedispersion was pipetted onto the square to cover the surface beforespinning the chuck at 2,500 rpm for 60 seconds in an attempt tospin-coat the aniline onto the ITO layer on the square. When thespinning was stopped, there was no trace of polyaniline deposited on thesquare. This illustrates the difficulty of depositing polyaniline on theflexible substrate with the ITO coating when no surfactant is used.

Example 1

This Example demonstrates the results obtained with a light-emittingdiode containing a buffer layer formed by spin-coating an aqueouspolyaniline dispersion to which has been added polyoxyethylene (10)isooctylphenyl ether surfactant onto a flexible substrate containing anITO anode.

A square of the flexible PET substrate with the ITO layer cleaned anddried in Comparative Experiment A was used for spin-coating the aqueouspolyaniline dispersion Ormecon® D1002W, 2.6% w/w polyaniline in water,to which was added Triton® X-100, polyoxyethylene (10) isooctylphenylether, a non-ionic surfactant (product # 234729, Sigma-Aldrich Corp.,St. Louis, Mo., USA). The concentration of Triton® X-100 was 0.35% w/wTriton® X-100 in Ormecon D1002W. Spin-coating of the aqueous dispersioncontaining the surfactant was carried out in essentially the same manneras described in Comparative Experiment A. The result was a smoothcoating of the electrically conductive polyaniline and surfactant on theITO layer. The coated-ITO-PET square was placed in a vacuum oven and thecoating dried overnight at room temperature before being spin-coatedwith 1.50% w/w polyfluorene in p-xylene at 2,500 rpm for 60 seconds. Thepolyfluorene, a green emitting material, was GB001 (Cambridge DisplayTechnology, Cambridge, UK) and serves as the active emission layer. Thecoated sample was placed in a vacuum oven at room temperature overnightbefore the vapor-deposition of the cathode, an array of aluminum dotshaving the dimensions of 1.0 mm in diameter and about 200 nm thick. Thiscompleted the fabrication of the light-emitting diode.

Light emission measurements were carried out on this flexiblelight-emitting diode as described above. The device had a turn-onvoltage of 3.5 V and a photodiode current of 320 nA when the devicevoltage and current were 5.3 V and 1 mA respectively. The emission wasalready visibly bright under room light when the photodiode current wasonly 100 nA. This Example illustrates use of an aqueous dispersion of anelectrically conducting polymer, polyaniline, to which has been added asurfactant, polyoxyethylene (10) isooctylphenyl ether to form a bufferlayer and the performance of that buffer layer in a polymericlight-emitting diode on a flexible substrate. The comparison of thisresult with that of Comparative Experiment A shows the advantage of thepresence of the surfactant.

Comparative Experiment B

This Comparative Experiment demonstrates the results obtained whenforming a buffer layer by spin-coating an aqueous polyaniline dispersionwith no surfactant onto a flexible substrate containing an ITO anode inan attempt to make a light-emitting diode.

The flexible substrate used in this Comparative Experiment isAlt-O-25-B-G-7 (180 μm) poly(ethylene terephthalate) PET sheet, on oneside of which has been deposited a thin layer of 25 ohm per square ITO,obtained from Southwall Technologies, Inc. (Palo Alto, Calif.). 1 inch(2.5 cm) square pieces cut from the sheet were cleaned and dried inessentially the same manner as described in Comparative Experiment A.Spin-coating of the aqueous polyaniline dispersion Ormecon® D1002W, 2.6%w/w polyaniline in water, was carried out in essentially the same manneras described in Comparative Experiment A. The aqueous dispersion formeda high contact angle on the substrate and therefore resulted in onlyspotty coverage on the ITO surface. Spin-coating the surface with anelectroluminescent polymer solution to form a cathode and complete thelight-emitting diode was not carried out due to the spotty coating ofthe polyaniline.

Example 2

This Example demonstrates the results obtained with a light-emittingdiode containing a buffer layer formed by spin-coating an aqueouspolyaniline dispersion to which has been added polyoxyethylene (10)isooctylphenyl ether surfactant onto a flexible substrate containing anITO anode.

A square of the flexible PET substrate with the ITO layer cleaned anddried in Comparative Experiment B was used for spin-coating the aqueouspolyaniline dispersion Ormecon® D1002W, 2.6% w/w polyaniline in water,to which was added Triton® X-100, polyoxyethylene (10) isooctylphenylether. The concentration of Triton® X-100 was 0.40% w/w Triton® X-100 inOrmecon® D1002W. Spin-coating of the aqueous dispersion containing thesurfactant was carried out in essentially the same manner as describedin Comparative Experiment A. The result was a smooth coating of theelectrically conductive polyaniline and surfactant on the ITO layer. Thefabrication of light-emitting diode was completed following theprocedure of Example 1. Light emission measurements were carried out onthis light-emitting diode essentially as described in Example 1. Thedevice had a turn-on voltage of 4.0 V and a photodiode current of 241 nAwhen the device voltage and current were 6.6 V and 1 mA respectively.The emission was visibly bright under room light when the photodiodecurrent was 100 nA. This Example illustrates use of an aqueousdispersion of an electrically conducting polymer, polyaniline, to whichhas been added a surfactant, polyoxyethylene (10) isooctylphenyl etherto form a buffer layer and the performance of that buffer layer in apolymeric light-emitting diode on a flexible substrate. The comparisonof this result with that of Comparative Experiment B shows the advantageof the presence of the surfactant.

Examples 3–5 and Comparative Experiments C–D

Examples 3–5 demonstrate the results obtained with PLEDs containingbuffer layers formed by spin-coating aqueous polyaniline dispersions towhich have been added various amounts of polyoxyethylene (10)isooctylphenyl ether surfactant onto flexible substrates containing ITOanodes. Comparative Experiment C demonstrates the results obtained witha light-emitting diode containing no buffer layer. ComparativeExperiment D demonstrates the results obtained with a light-emittingdiode containing a buffer layer formed by spin-coating an aqueouspolyaniline dispersion with no surfactant onto a flexible substratecontaining an anode.

The flexible substrate used for these Examples and ComparativeExperiments was NV1-062/ST504 5 mil (130 μm) poly(ethyleneterephthalate) (PET) sheet on one side of which has been deposited athin layer of 40 ohm per square ITO, obtained from NeoVac (Santa Rosa,Calif.). 1 inch squares of the sheet were cleaned and dried inessentially the same manner as described in Comparative Experiment A.The aqueous polyaniline dispersion used in these Examples andComparative Experiment D is Ormecon® D1002W, 2.6% w/w polyaniline inwater. The dispersion used to spin-coat the buffer layer of ComparativeExperiment D contained no surfactant. The dispersions used to spin-coatthe buffer layers of Examples 3, 4 and 5 had concentrations ofsurfactant of 0.05, 0.1 and 0.35% w/w Triton® X-100, polyoxyethylene(10) isooctylphenyl ether, in Ormecon® D1002W, respectively.Spin-coating of the aqueous dispersions containing the surfactant andthat without the surfactant were carried out in essentially the samemanner as described in Comparative Experiment A. The fabrication of thelight-emitting diode was completed following the procedure of Example 1.The square for Comparative Experiment C contained no buffer layer sothat the spin-coating of the polyfluorene layer was done directly ontothe ITO layer The polyaniline dispersion without the surfactant did notwet the surface sufficiently in Comparative Example D to provide acomplete, smooth coverage of the surface and therefore resulted in aspotty and inhomogeneous buffer layer. However, the active emissionlayer and the aluminum dot cathode were deposited in an attempt tofabricate a light-emitting diode.

Light emission measurements were carried out as described above and theresults are shown in Table I.

TABLE I Photodiode Device Current (μA) Voltage (V) Turn-on @ 1 mA @ 1 mAExample or Buffer Layer Voltage device device Comp. Expt. Quality (V)current current Comparative No buffer 5.5 0.62 10.5 Experiment C LayerComparative Inhomogeneous 4.0 1.07, 0.72 5.8, 8.2 Experiment D Example 3Homogeneous 4.0 0.90 5.6 Example 4 Homogeneous 3.5 1.10 5.6 Example 5Homogeneous 4.0 0.66 5.5

The PLED without a buffer layer of Comparative Experiment C had a higherturn-on voltage and operating voltage then those of ComparativeExperiment D and Examples 3–5, all of which have buffer layers. The datashown in Table I for Comparative Experiment D are the results for two ofthe dots of the cathode. One had a photodiode current of 1.07 μA whenthe device voltage and current were 5.8 V and 1 mA respectively. Theother had a photodiode current of 0.72 μA when the device voltage andcurrent were 8.2 V and 1 mA respectively. The variability is a result ofthe inhomogeneity of the buffer layer that results when no surfactant isused. The PLEDs of Examples 3–5 all had lower turn-on voltages, higherphotodiode currents and lower operating voltages than the PLED ofComparative Experiment C and show consistency from one cathode dot toanother in contrast to the PLED of Comparative Experiment D. Thephotodiode current increased as the amount of surfactant increased from0.05% w/w surfactant in the aqueous dispersion in Example 3 to 0.1% w/wsurfactant in the aqueous dispersion in Example 4 and then decreased asthe amount of surfactant was increased to 0.35% w/w surfactant in theaqueous dispersion in Example 5. To get the greatest improvement inbuffer layer performance when polyoxyethylene (10) isooctylphenyl ethersurfactant is added to the aqueous polyaniline dispersion of theseExamples, the amount of surfactant used should be at least 0.05% w/wsurfactant in the aqueous dispersion and preferably less than 0.30% w/wsurfactant in the aqueous dispersion.

Examples 6–8 and Comparative Experiments E–G

These Examples and Comparative Experiments illustrate the amount ofpolyoxyethylene (10) isooctylphenyl ether surfactant that must be addedto an aqueous polyaniline dispersion to provide the surface tensionrequired for coatability of a buffer layer on a flexible substrate thatwill result in improved performance.

The aqueous polyaniline dispersion used in these Examples andComparative Experiments is Ormecon® D1002W, 2.6% w/w polyaniline inwater. The dispersions of Comparative Experiment E, Examples 6–8 andComparative Experiments F and G had concentrations of surfactant of 0.0.05, 0.10, 0.16, 0.5 and 1.0% w/w Triton® X-100, polyoxyethylene (10)isooctylphenyl ether, in Ormecon® D1002W, respectively.

The surface tension was measured as described above. The surfacetensions of dispersions of Comparative Experiment E, Examples 6–8 andComparative Experiments F and G were 41.9, 34.1, 33.7 and 33.3, 31.9 and31.8 mN/m, respectively. There was little change in surface tension forsurfactant concentrations above that of Example 8, i.e., above 0.16 w/wsurfactant in the aqueous dispersion. The amount of this surfactant thatshould be used with this aqueous dispersion is at least about 0.05% w/wsurfactant in the aqueous dispersion and less than about 0.40% w/wsurfactant in the aqueous dispersion, preferably less than about 0.30%w/w surfactant in the aqueous dispersion. Higher concentrations ofsurfactant would adversely impact device performance as illustrated inExample 3–5 and Comparative Experiments C–D.

Examples 9–11 and Comparative Experiment H

These Examples and Comparative Experiment illustrate the amount ofpolyoxyethylene (10) isooctylphenyl ether surfactant that must be addedto an aqueous poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)(PEDOT/PSS) dispersion to provide the surface tension required forcoatability of a buffer layer on a flexible substrate that will resultin improved performance.

The aqueous PEDOT/PSS dispersion used in these Examples and ComparativeExperiment is Baytron®-P (VP Al4083) obtained from Bayer AG (Germany).The Baytron®-P contains 1.5 to 2.0% w/wpoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) inwater. The dispersions of Comparative Experiment H and Examples 9–11 hadconcentrations of surfactant of 0. 0.05, 0.10 and 0.15% w/w Triton®X-100, polyoxyethylene (10) isooctylphenyl ether, in the aqueousPEDOT/PSS dispersion, respectively.

The surface tension was measured as described above. The surfacetensions of dispersions of Comparative Experiment H and Examples 9–11were 42.6, 29.7, 30.4 and 30.8 mN/m, respectively. There was littlechange in surface tension for surfactant concentrations above that ofExample 9, i.e., above 0.05% w/w surfactant in the aqueous dispersion.The amount of this surfactant that should be used with this aqueousdispersion is at least about 0.05% w/w surfactant in the aqueousdispersion and less than about 0.40% w/w surfactant in the aqueousdispersion, preferably less than about 0.30% w/w surfactant in theaqueous dispersion.

Examples 12–16 and Comparative Experiments I, J

Examples 12–16 and Comparative Experiments I and J demonstrate theresults obtained with PLEDs containing buffer layers formed byspin-coating aqueouspolyaniline/poly(2-acrylamido-2-methyl-1-propanesulfonic acid)(PAni/PAAMPSA) dispersions to which have been added certain surfactantsonto glass substrates containing ITO anodes.

The glass substrates were 3 cm×3 cm with a 1.5 cm×2.0 cm layer of ITOhaving a thickness of 100 to 150 nm. These substrates with the ITOlayers were cleaned and dried in essentially the same manner asdescribed in Comparative Experiment A and subsequently treated withoxygen plasma to purposely remove the factor of wetability forcomparison of light emission properties with the various surfactants.

The aqueous dispersion used to spin-coat a buffer layer was 1.0% w/welectrically conductivepolyaniline/poly(2-acrylamido-2-methyl-1-propanesulfonic acid),PAni/P(AAMPSA), in water obtained from Uniax Corp. (Santa Barbara,Calif.).

No surfactant was added to the aqueous dispersion used to spin-coat thebuffer layer of Comparative Experiment I. The surfactant added to theaqueous dispersion used to spin-coat the buffer layer of ComparativeExperiment J was lithium dodecylsulfate, an ionic surfactant (Cat#86,190-1, Sigma-Aldrich Corp., St. Louis, Mo., USA). The concentrationof lithium dodecylsulfate was 0.15% w/w lithium dodecylsulfate in theaqueous PAni/P(AAMPSA) dispersion. The surfactant added to the aqueousdispersion used to spin-coat the buffer layer of Example 12 was Triton®X-100, polyoxyethylene (10) isooctylphenyl ether. The concentration ofTriton® X-100 was 0.15% w/w Triton® X-100 in the aqueous PAni/P(AAMPSA)dispersion. The surfactant added to the aqueous dispersion used tospin-coat the buffer layer of Example 13 was Zonyl® FS-300, a non-ionicfluorosurfactant containing a perfluoroalkylpolyoxyethylene with theformula R_(f)CH₂CH₂O(CH₂CH₂O)_(z)H wherein R_(f) is a perfluoroalkylgroup with the formula F(CF₂CF₂)₃₋₈ (DuPont Co., Wilmington, Del.). Theconcentration of Zonyl® FS-300 was 0.15% w/w Zonyl® FS-300 in theaqueous PAni/P(AAMPSA) dispersion. The surfactant added to the aqueousdispersion used to spin-coat the buffer layer of Example 14 was Zonyl®FSN-100, a non-ionic fluorosurfactant containing aperfluoroalkylpolyoxyethylene with the formulaR_(f)CH₂CH₂O(CH₂CH₂O)_(x)H wherein R_(f) is a perfluoroalkyl group withthe formula F(CF₂CF₂)₃₋₈ (DuPont Co., Wilmington, Del.). Theconcentration of Zonyl® FSN-100 was 0.15% w/w Zonyl® FSN-100 in theaqueous PAni/P(AAMPSA) dispersion. The surfactant added to the aqueousdispersion used to spin-coat the buffer layer of Example 15 was 3M®Fluorad® Fluorosurfactant FC430, a non-ionic fluorosurfactant containinga fluoroaliphatic polymeric ester (3M, St. Paul, Minn.). Theconcentration of FC430 was 0.05% w/w FC430 in the aqueous PAni/P(AAMPSA)dispersion. The surfactant added to the aqueous dispersion used tospin-coat the buffer layer of Example 16 was Zonyl® FSA, an ionicfluorosurfactant containing a lithium carbonate of aperfluoroalkylethylenesulfurethylene with the formulaR_(f)CH₂CH₂SCH₂CH₂CO₂Li wherein R_(f) is a perfluoroalkyl group with theformula F(CF₂CF₂)₃₋₈ (E. I. du Pont de Nemours and Company, Wilmington,Del.). The concentration of Zonyl® FSA was 0.10% w/w Zonyl® FSA in theaqueous PAni/P(AAMPSA) dispersion.

Each aqueous dispersion with or without a surfactant was spin-coatedonto the ITO containing glass substrates at a spinning speed of 1200rpm. The buffer layer thicknesses were in the range of 50 nm; thethickness of each is shown in Table II. The ITO containing glasssubstrates with the buffer layers were dried in nitrogen at 90° C. for30 minutes.

The buffer layers were each then spin-coated with an active emissionlayer comprised of a yellow emitter PDY® 131, apoly(substituted-phenylene vinylene) (Covion Company, Frankfurt,Germany) in essentially the same manner as described for spin-coatingthe active emission layer of Example 1. The thickness of the activeemission layer was approximately 70 nm. All film thickness was measuredwith a TENCOR 500 Surface Profiler (TENCOR, Inc., Mountain View,Calif.).

To serve as the cathode for each of the PLEDs, first a layer of bariumand then a layer of aluminum were vapor deposited on top of the activeemission layer under a vacuum of 1×10⁻⁶ torr. The thickness of thebarium layer was 3 nm; the thickness of the aluminum layer was 300 nm.

Device performance was tested as described above, with the results shownin Table II.

TABLE II Brightness Buffer Efficiency (Cd/m²) Layer Voltage (V) (Cd/A) @3.3 Example or Thickness @ 200 Cd/m² @ 200 Cd/m² mA/cm², Comp. Expt.(nm) Brightness Brightness 80° C. Comparative 59.8 3.3 6.7 204Experiment I Comparative 44.0 electrical short — — Experiment J Example12 48.7 3.2 7.6 234 Example 13 44.6 3.2 7.6 240 Example 14 57.4 3.2 8.0200 Example 15 52.4 3.5 7.5 — Example 16 47.8 3.2 7.2 —

The lithium dodecylsulfate surfactant used in the buffer layer ofComparative Experiment J resulted in an electrical short in the PLEDwhich indicates that alkylsulfate surfactants should not be used inmaking the buffer layer.

The PLED of Example 12, with a buffer layer made with polyoxyethylene(10) isooctylphenyl ether surfactant added to the aqueous PAni/P(AAMPSA)dispersion, showed a higher light emission efficiency at 200 Cd/m², acomparable turn-on voltage at 200 Cd/M², and higher light emissionbrightness at 3.3 mA/cm² and 80° C. as compared with the PLED ofComparative Experiment I, with a buffer layer made without a surfactantbeing added to the aqueous PAni/P(AAMPSA) dispersion.

The PLEDs of Examples 13–16, with buffer layers made with afluorosurfactant added to the aqueous PAni/P(AAMPSA) dispersion, showeda higher light emission efficiency at 200 Cd/m², a slightly lower orcomparable turn-on voltage at 200 Cd/m², and equivalent or higher lightemission brightness at 3.3 mA/cm² and 80° C. as compared with the PLEDof Comparative Experiment I, with a buffer layer made without asurfactant being added to the aqueous PAni/P(AAMPSA) dispersion.

These Examples and Comparative Experiments demonstrate the improvementin PLED performance obtained with PLEDs containing buffer layers formedby spin-coating aqueouspolyaniline/poly(2-acrylamido-2-methyl-1-propanesulfonic acid)dispersions to which have been added a polyoxyethylene (10)isooctylphenyl ether surfactant, a fluorosurfactant containing afluoroaliphatic polymeric ester or a fluorosurfactant containing aperfluoroalkyl group with the formula F(CF₂CF₂)₃₋₈.

Examples 17–20 and Comparative Experiments K, L

Examples 17–20 and Comparative Experiments K and L demonstrate theresults obtained with PLEDs containing buffer layers formed byspin-coating aqueouspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS)dispersions to which have been added various surfactants onto glasssubstrates containing ITO anodes.

The aqueous (PEDOT/PSS) dispersion used to spin-coat a buffer layer wasBaytron-P® which contains 1.5 to 2.0% w/wpoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) inwater and was obtained from Bayer AG (Germany).

No surfactant was added to the aqueous dispersion used to spin-coat thebuffer layer of Comparative Experiment K. The surfactant added to theaqueous dispersion used to spin-coat the buffer layer of ComparativeExperiment L was lithium dodecylsulfate. The concentration of lithiumdodecylsulfate was 0.15% w/w lithium dodecylsulfate in the aqueousPEDOT/PSS dispersion. The surfactant added to the aqueous dispersionused to spin-coat the buffer layer of Example 17 was Triton® X-100 witha concentration of 0.15% w/w Triton® X-100 in the aqueous PEDOT/PSSdispersion. The surfactant added to the aqueous dispersion used tospin-coat the buffer layer of Example 18 was Zonyl® FS-300 with aconcentration of 0.15% w/w Zonyl® FS-300 in the aqueous PEDOT/PSSdispersion. The surfactant added to the aqueous dispersion used tospin-coat the buffer layer of Example 19 was 3M® Fluorad®Fluorosurfactant FC-430 with a concentration of 0.05% w/w FC-430 in theaqueous PEDOT/PSS dispersion. The surfactant added to the aqueousdispersion used to spin-coat the buffer layer of Example 20 was Zonyl®FSA with a concentration of 0.10% w/w Zonyl® FSA in the aqueousPEDOT/PSS dispersion.

Light-emitting diodes were fabricated using the same substrate materialand procedures as described in Examples 12–16. The devices were testedas described for Examples 12–16, with the results shown in Table IIIbelow.

TABLE III Brightness Buffer Efficiency (Cd/m²) Layer Voltage (V) (Cd/A)@ 3.3 Example or Thickness @ 200 Cd/m² @ 200 Cd/m² mA/cm², Comp. Expt.(nm) Brightness Brightness 80° C. Comparative 135 3.3 7.0 202 ExperimentK Comparative 137 3.2 5.7 140 Experiment L Example 17 154 3.4 7.3 270Example 18 120 3.3 7.5 217 Example 19 120 3.2 7.7 212 Example 20 120 3.47.6 194

The PLED of Comparative Experiment L, with a buffer layer made withlithium dodecylsulfate surfactant added to the aqueous PEDOT/PSSdispersion, showed a lower light emission efficiency at 200 Cd/m², acomparable turn-on voltage at 200 Cd/m², and lower light emissionbrightness at 3.3 mA/cm² and 80° C. as compared with the PLED ofComparative Experiment K, with a buffer layer made without a surfactantbeing added to the aqueous PEDOT/PSS dispersion. This demonstrates againthat alkylsulfate surfactants should not be used in making the bufferlayer.

The PLED of Example 17, with a buffer layer made with polyoxyethylene(10) isooctylphenyl ether surfactant added to the aqueous PEDOT/PSSdispersion, showed a higher light emission efficiency at 200 Cd/m², acomparable turn-on voltage at 200 Cd/m², and higher light emissionbrightness at 3.3 mA/cm² and 80° C. as compared with the PLED ofComparative Experiment J, with a buffer layer made without a surfactantbeing added to the aqueous PEDOT/PSS dispersion.

The PLEDs of Examples 18 and 20, with buffer layers made withfluorosurfactants containing a perfluoroalkyl group with the formulaF(CF₂CF₂)₃₋₈ added to the aqueous PEDOT/PSS dispersion, showed a higherlight emission efficiency at 200 Cd/m², a comparable turn-on voltage at200 Cd/m², and higher or comparable light emission brightness at 3.3mA/cm² and 80° C. as compared with the PLED of Comparative Experiment K,with a buffer layer made without a surfactant being added to the aqueousPEDOT/PSS dispersion. The PLED of Example 19, with a buffer layer madewith the fluorosurfactant containing a fluoroaliphatic polymeric esteradded to the aqueous PEDOT/PSS dispersion, showed a higher lightemission efficiency at 200 Cd/m², a comparable turn-on voltage at 200Cd/m², and higher light emission brightness at 3.3 mA/cm² and 80° C. ascompared with the PLED of Comparative Experiment J, with a buffer layermade without a surfactant being added to the aqueous PEDOT/PSSdispersion.

These Examples and Comparative Experiments demonstrate the improvementin PLED performance obtained with PLEDs containing buffer layers formedby spin-coating aqueouspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) dispersions towhich have been added a polyoxyethylene (10) isooctylphenyl ethersurfactant, a fluorosurfactant containing a fluoroaliphatic polymericester or a fluorosurfactant containing a perfluoroalkyl group with theformula F(CF₂CF₂)₃₋₈.

Comparative Experiment M

This Comparative Experiment demonstrates the results obtained in anattempt to make a light-emitting diode with a bilayer anode comprised ofa metal layer and a dielectric layer on a flexible substrate and withoutthe buffer layer of the invention.

The flexible substrate used in this Comparative Experiment is atransparent conductive film Altair®-M 10 (SouthWell Company, Palo Alto,Calif.) 7-mil (180 μm) poly(ethylene terephthalate) (PET) sheet. On oneside of this sheet there has been deposited first a dielectric layer. Ametal layer, shown by energy dispersive X-ray spectroscopy to be agold/silver alloy, has been deposited onto the dielectric. An indiumoxide dielectric layer has been deposited onto the metal layer. It has asheet resistance of 10 ohms per square.

A 12 inch×1 inch (30.5 cm×2.5 cm) strip of the PET with the bilayeranode was cut into 1 inch (2.5 cm) squares. These squares were cleanedand dried in essentially the same manner as described in ComparativeExperiment A.

Devices were fabricated following the procedure of Example 1, with thesurface of the substrate with the dielectric/metal/dielectric facing up,and spinning 8 drops of polyfluorene for 90 seconds.

Light emission measurements were carried out on this light-emittingdiode essentially as described in Example 1. As indicated in Table IV,the device had a turn-on voltage greater than 15 V. As the voltage wasincreased above 15 V, the aluminum dot cathode burned before there wasany evidence of light emission. This Comparative Experiment shows thatthe flexible substrate with the bilayer anode and no buffer layer doesnot result in a light-emitting diode.

Comparative Experiment N

This Comparative Experiment demonstrates the results obtained in anattempt to make a light-emitting diode with a bilayer anode comprised ofa metal layer and a dielectric layer on a flexible substrate and with abuffer layer formed by spin-coating an aqueous polyaniline dispersionwith no surfactant onto the bilayer anode. A square of the flexible PETsubstrate with the bilayer anode cleaned and dried in ComparativeExperiment M was not wettable by an aqueous polyaniline dispersionOrmecon® D1002W, 2.6% w/w polyaniline in water, and a buffer layer couldnot be formed by spin coating, in essentially the same manner asdescribed in Comparative Example A, the polyaniline dispersion with nosurfactant present in the dispersion.

Example 21

This Example demonstrates the results obtained with a light-emittingdiode with a bilayer anode comprised of a metal layer and a dielectriclayer on a flexible substrate and a buffer layer formed by spin-coatingon to the anode an aqueous polyaniline dispersion to which has beenadded polyoxyethylene (10) isooctylphenyl ether surfactant.

Light-emitting diodes were fabricated following the procedure ofComparative Experiment M. Light emission measurements were carried outon this light-emitting diode essentially as described in Example 1, withthe results shown in Table IV The comparison of this result with that ofComparative Experiments M and N shows that the buffer layer of theinvention makes possible the use of a bilayer anode on this flexiblesubstrate in a light-emitting diode.

Comparative Experiment O

This Comparative Experiment demonstrates the with a light-emitting diodewith a bilayer anode comprised of a metal layer and a dielectric layeron a flexible substrate and without the buffer layer of the invention.

The flexible substrate used in this Comparative Experiment is atransparent conductive film AgHT4 PET (CPFilms Inc., Canoga Park,Calif.), a 7-mil (180 μm) poly(ethylene terephthalate) (PET) sheet. Onone side of this sheet there has been deposited first a thin layer ofsilver. A dielectric layer has been deposited onto the silver layer. Ithas a sheet resistance of 4 ohms per square.

A 6 inch×1 inch (15.2 cm×2.5 cm) strip of the PET with the bilayer anodewas cut into 1 inch (2.5 cm) squares. These squares were cleaned anddried in essentially the same manner as described in ComparativeExperiment A.

A light-emitting diode was fabricating following the procedure ofComparative Experiment M.

Light emission measurements were carried out on this light-emittingdiode essentially as described in Example 1, with the results shown inTable IV. This Comparative Experiment shows that the flexible substratewith the bilayer anode results in a poor performing light-emitting diodewithout the buffer layer of the invention.

Comparative Experiment P

This Comparative Experiment demonstrates the results obtained in anattempt to make a light-emitting diode with a bilayer anode comprised ofa metal layer and a dielectric layer on a flexible substrate and with abuffer layer formed by spin-coating an aqueous polyaniline dispersionwith no surfactant onto the bilayer anode.

A square of the flexible PET substrate with the bilayer anode cleanedand dried in Comparative Experiment O was not wettable by an aqueouspolyaniline dispersion Ormecon® D1002W, 2.6% w/w polyaniline in water,and a buffer layer could not be formed by spin coating, in essentiallythe same manner as described in Comparative Example A, the polyanilinedispersion with no surfactant present in the dispersion.

Example 22

This Example demonstrates the results obtained with a light-emittingdiode with a bilayer anode comprised of a metal layer and a dielectriclayer on a flexible substrate and a buffer layer formed by spin-coatingon to the anode an aqueous polyaniline dispersion to which has beenadded polyoxyethylene (10) isooctylphenyl ether surfactant.

A square of the flexible PET substrate with the bilayer anode cleanedand dried in Comparative Experiment O was used to prepare alight-emitting diode essentially as described in Example 21 except thatthe concentration of Triton® X-100 was less than 0.3% w/w Triton® X-100in Ormecon® D1002W.

Light emission measurements were carried out on this light-emittingdiode essentially as described in Example 1, with the results shown inTable IV. The comparison of this result with that of ComparativeExperiments O and P shows that the buffer layer of the invention makespossible the use of a bilayer anode on this flexible substrate in alight-emitting diode.

TABLE IV Photodiode Device Turn-on Current (nA) Voltage (V) Example orVoltage @ 1 mA device @ 1 mA device Comp. Expt. (V) current currentComparative >15 0 — Experiment M Example 21 4.0 203 6.3 Comparative 1460 24 Experiment O Example 22 4.0 300 6.7

1. A light-emitting device on a substrate, said light-emitting device comprising: (a) an anode; (b) a cathode; (c) an active emission layer positioned between said anode and said cathode; and (d) a buffer layer positioned between said anode and said active emission layer, wherein said buffer layer is comprised of an electrically conducting polymer and a surfactant, the amount of said surfactant is from about 0.5 wt % to about 15 wt % of the total weight of said surfactant and said electrically conducting polymer, and said surfactant is a fluorosurfactant or a polyoxyethylene alkylphenol ether.
 2. The device of claim 1, wherein said surfactant is a non-ionic surfactant.
 3. The device of claim 2, wherein said non-ionic surfactant is a fluorosurfactant.
 4. The device of claim 3, wherein said fluorosurfactant contains a perfluoroalkyl group with the formula F(CF₂CF₂)₃₋₈.
 5. The device of claim 4, wherein said fluorosurfactant is a perfluoroalkylpolyoxyethylene.
 6. The device of claim 3, wherein said fluorosurfactant is comprised of a fluoroaliphatic polymeric ester.
 7. The device of claim 2, wherein said non-ionic surfactant is a polyoxyethlyene alkylphenol ether.
 8. The device of claim 7, wherein said polyoxyethlyene alkylphenol ether is polyoxyethylene (10) isooctylphenyl ether.
 9. The device of claim 1 wherein said fluorosurfactant is an ionic fluorosurfactant.
 10. The device of claim 1, wherein the amount of said surfactant is from about 1 wt % to about 10 wt % of the total weight of said surfactant and said electrically conducting polymer.
 11. The device of claim 1, wherein said electrically conducting polymer is a polyaniline.
 12. The device of claim 1, wherein said electrically conducting polymer is a poly(dioxythiopene).
 13. The device of claim 12, wherein said poly(dioxythiopene) is poly(3,4-ethylenedioxythiophene).
 14. The device of claim 1, wherein said anode is comprised of at least one bilayer comprised of a metal layer and a dielectric layer and wherein at least one of said dielectric layer is adjacent to said buffer layer.
 15. The device of claim 14, wherein at least one of said dielectric layer disposed adjacent to said buffer layer is disposed directly on said buffer layer.
 16. A light-emitting device on a substrate, said light-emitting device comprising: (a) an anode; (b) a cathode; (c) an active emission layer positioned between said anode and said cathode; and (d) a buffer layer positioned between said anode and said active emission layer, wherein said buffer layer is formed using an aqueous dispersion of an electrically conducting polymer to which has been added a surfactant, said dispersion is characterized by a surface tension of between 25 and 35 mN/M, the concentration of said electrically conducting polymer is from 0.1 to 5.0% w/w of said electrically conducting polymer in said dispersion, the concentration of said surfactant is less than about 0.4% w/w of said surfactant in said dispersion, and said surfactant is a fluorosurfactant or a polyoxyethylene alkylphenol ether.
 17. The device of claim 16, wherein said surfactant is a non-ionic surfactant.
 18. The device of claim 17, wherein said non-ionic surfactant is a fluorosurfactant.
 19. The device of claim 18, wherein said fluorosurfactant contains a perfluoroalkyl group with the formula F(CF₂CF₂)₃₋₈.
 20. The device of claim 19, wherein said fluorosurfactant is a perfluoroalkylpolyoxyethylene.
 21. The device of claim 18, wherein said fluorosurfactant is comprised of a fluoroaliphatic polymeric ester.
 22. The device of claim 17, wherein said non-ionic surfactant is a polyoxyethlyene alkylphenol ether.
 23. The device of claim 22, wherein said polyoxyethlyene alkylphenol ether is polyoxyethylene (10) isooctylphenyl ether.
 24. The device of claim 16, wherein said concentration of said electrically conducting polymer is from 0.1 to 5.0% w/w of said electrically conducting polymer in said dispersion and said concentration of said surfactant is less than about 0.3% w/w of said surfactant in said dispersion.
 25. The device of claim 16, wherein said electrically conducting polymer is a polyaniline.
 26. The device of claim 16, wherein said electrically conducting polymer is a poly(dioxythiopene).
 27. The device of claim 26, wherein said poly(dioxythiopene) is poly(3,4-ethylenedioxythiophene).
 28. The device of claim 16, wherein said anode is comprised of indium tin oxide.
 29. The device of claim 16, wherein said anode is comprised of at least one bilayer, each of the at least one bilayer comprising a metal layer and a dielectric layer, wherein at least one of said dielectric layer is adjacent to said buffer layer.
 30. The device of claim 29, wherein at least one of said dielectric layer disposed adjacent to said buffer layer is disposed directly on said buffer layer. 